There is an increasing demand for alkylate fuel blending feedstock. As a fuel-blending component alkylate combines a low vapour pressure, no sulphur, olefins or aromatics with high octane properties.
Almost all alkylate is produced by reacting isobutane with butene in the presence of a suitable acidic catalyst. The most used catalysts are HF and sulphuric acid, although other catalysts such a solid acid catalyst have been reported. Recently, the alkylation of isoparaffins with olefins using an acidic ionic liquid catalyst has attracted attention as an alternative to HF and sulphuric acid catalysed alkylation processes.
In for instance U.S. Pat. No. 7,285,698 a process for manufacturing an alkylate oil is disclosed, which uses a composite ionic liquid catalyst to react isobutane with a butene. In the process of U.S. Pat. No. 7,285,698, isobutane and butene are supplied to a reactor and the alkylate is formed by contacting the reactants with a composite ionic liquid under alkylation conditions. The reactor effluent is separated and the ionic liquid phase is recycled to the reactor while the hydrocarbon phase is treated to retrieve the alkylate. A disadvantage of the process of U.S. Pat. No. 7,285,698 is that the ratio of isobutane to butene in the feed to the process is too low when using ionic liquid catalyst. Therefore, a large hold up of isobutane in the reactor is required to provide a high isobutane to butene ratio.
In CN1012449752, a method for preparing an alkylate is disclosed wherein in a reactor isobutane is reacted with a liquid olefin in the presence of an ionic liquid catalyst to obtain an alkylate. In CN1012449752, a high isobutane to olefin ratio in the reactor is maintained by recycling part of the hydrocarbons, including isobutane, in the reactor effluent. Separately, the catalyst is recycled to the reactor. In CN1012449752, a raw mixture of isobutane and olefins having a low isobutane to olefin ratio is fed to the reactor. This raw mixture is directly provided into the reactor resulting in localised high olefin concentrations in the reactor. Such high olefin concentrations induce side reactions such as olefin polymerisation.